What's killing Cape's salt marshes?

A small purple crab is munching away at Cape Cod's vulnerable marshes, thriving on unintended consequences from actions that go back nearly a century, according to a research paper released in January.

A small purple crab is munching away at Cape Cod's vulnerable marshes, thriving on unintended consequences from actions that go back nearly a century, according to a research paper released in January.
“It looks like you just mowed your lawn,” said Tyler Coverdale, one of the authors of the paper, which was published in the scientific journal Frontiers in Ecology and the Environment.
Coverdale, a laboratory manager at the Department of Ecology and Evolutionary Biology at Brown University, and the paper's other authors used aerial photographers from late 1939 through 2005 and geographic information systems technology to analyze marsh die-off around the Cape.
The researchers found that die-off was most pronounced in areas where two factors were present: ditches dug before the 1930s to drain mosquito breeding habitat and intense shoreline development.
“Development ultimately is what causes the die-off to happen,” Coverdale said.
Marinas, dredging for large boats and the construction of seashore homes with private docks provided access to fishermen that didn't previously exist, he said.

What leads to marsh die-off (click to enlarge):

Increased fishing reduced the population of striped bass, blue crabs and other predators of the purple marsh crab, he said.
“That purple marsh crab eats one species of grass,” he said, adding that the voracious inch-wide crab prefers cordgrass found along the water's edge in areas like those around the mosquito ditches. “That's basically just creating more habitat.”
As the crabs devour the cordgrass and burrow into the muddy bank, they destabilize the edge of the marsh, increasing erosion and further widening the ditches, Coverdale said.
The paper released by Coverdale and his co-authors last month is one in a series they have done analyzing the impact of the crabs on salt marsh habitat. It is the first that makes the direct connection between ditch-digging, development and die-off.
The mosquito ditches were dug as part of a public works project in the early 1900s. They were intended to provide jobs and make areas like Cape Cod more livable.
Although the Cape Cod Mosquito Control Project maintains ditches in the region, the creation of the ditches occurred before the program was established, said Gabrielle Sakolsky, staff entomologist with the project.
“He could be right,” Sakolsky said about Coverdale's hypothesis that the combination of ditch-digging and development has boosted crab populations.
Each year, the project cleans between 30,000 and 40,000 feet of salt marsh ditches, she said.
The alternatives to maintaining the ditches and applying larvicide to trouble spots include aerial spraying for mosquitoes, which can be more expensive and is often less palatable to the public than the project's targeted treatment and ditch work, Sakolsky said.
“But we would not want to do anything that would cause any harm, obviously,” she said, adding that if the clearing efforts were contributing directly to erosion, the project's workers would notice.
Coverdale's model makes sense, but there are still too many unanswered questions to accept it as gospel, said plant ecologist Stephen Smith at the Cape Cod National Seashore.

How the shoreline has changed (click to enlarge):

“There's marshes on Cape Cod that have significant die-back that are in totally undeveloped places,” Smith said.
A marsh near Jeremy Point off Wellfleet, for example, is far removed from development but is experiencing significant die-off, he said.
In other marshes in the Mid-Cape and Upper Cape where there is a lot of development nearby, there is no die-off, Smith said.
Smith, who has studied the factors behind marsh die-off extensively, said he doesn't necessarily take issue with the hypothesis in the paper but would have liked to see it address exceptions to the rule.
“It very well may be true,” Smith said.
Other factors may affect the population of purple marsh crabs, which are missing altogether from some marshes, he said.
One thing that is hard to argue: The crab's diet is the direct cause of die-off, Smith said.
The mosquito ditches provide the crabs with access to a lot of habitat that they normally wouldn't have, he said, adding that there are other problems associated with the ditches.
On balance, however, the ditches provide other services beyond mosquito control, including buffering the marshes from the effects of climate change, Smith said.
“If we filled them all in, we'd drown all the marshes because of sea level rise,” he said.
Marshes are important for juvenile fish, which use the habitat for protection from predation, Coverdale said.
With the marsh die-off, those benefits are being lost, he said.
“You very well could have die-off in an area where you could look around and say, 'I don't see why people would fish here,'” Coverdale said, adding, however, that fishing is inevitably occurring.
Given the benefits provided by ditches and the alternatives to keeping them clear, the best solution to controlling the crab populations seems to lie in boosting the population of their predators, Coverdale said.
“That's the solution that we see being the most sustainable,” he said.